The present invention relates to an automatic contrast correction method and apparatus thereof, and more particularly to a method and apparatus for automatically correcting contrast to maintain optimum contrast of a reproduced picture for use in a color video printer.
A color video printer is a printing apparatus which prints on paper still or captured pictures that are selected or captured from moving pictures photographed by camera recorders ("camcorders"). s A conventional color video printer comprises a first decoder for converting a reproduced video signal into color difference signals, memories for storing color difference signal data of the pictures to be printed, a second decoder for converting the color difference signal data supplied from the memories into R, G and B signals, an encoder for converting the R, G and B signals into a composite video signal and supplying it to a monitor, line memory means in which the R, G and B signals are sequentially selected by a three-input-to-one output switch and the selected signals are converted into digital data so as to be stored in line memories, an intermediate gradation converter for converting the data supplied from the line memory means into a physical signal, and a thermal printing head (TPH) in which a heat generation amount is adjusted by an output of the intermediate gradation converter.
FIG. 1 is a block diagram of a color signal processor in a conventional color video printer.
In FIG. 1, a switch SW3 includes three inputs e, f and g, which receive the respective different color signals from a decoder (not shown) for converting the color difference signal data of a user's desired still picture or moving picture to be printed. That is, inputs e, f and g are B, G and R signals, respectively. Then, switch SW3 sequentially selects the R, G and B signals by a switch control signal CTL2 and outputs the selected signal. First, if the B signal is input through input e in switch SW3, it is sampled by a clock CLK in an A/D converter 18. That is, as shown in FIG. 2, a pulse is generated by clock CLK at the location of ".alpha.," so that all data (.alpha..sub.1, .alpha..sub.2, . . . , .alpha..sub.525) at the location thereof is sampled by the clock pulse. That is, the clock pulses are in time coincidence with the positions .alpha., . . . .alpha..sub.25 of a roster type scan of the picture to be presented; said picture being presented in the form of B, G and R signals input to switch SW3. The sampled data is stored in one of two line memories through a switch SW4. First and second line memories 41 and 42 are controlled by a line memory controller 43. Switches SW4 and SW5 are simultaneously controlled by a switch control signal CTL3. If terminals h and j are respectively selected in switches SW4 and SW5, the sampled data output from A/D converter 18 passes through switch SW4 and is stored in a first line memory 41 under the control of line memory controller 43. Then, the data which is stored in first line memory 41 passes through switch SW5 under the control of line memory controller 43 and is supplied to an intermediate gradation converter 19. Then, intermediate gradation converter 19 outputs a signal having a pulse width that varies according to the digital data input signal supplied from switch SW5 and supplies it to TPH 20. TPH 20 regulates a heat generation amount according to the signal supplied from intermediate gradation circuit 19. As described above, the color signal data at the location of ".alpha." which is sampled in A/D converter 18 is supplied to intermediate gradation converter 19 and TPH 20 through first line memory 41, thereby being printed on the paper, etc.
After the color signal data of a first line which is sampled at the location of ".alpha." is completely printed, the clock pulse is input at the location of ".beta.", so that all data (.beta..sub.1, .beta..sub.2, . . . , .beta..sub.525) at the location of ".beta.". is sampled. If terminals i and k are respectively selected in switches SW4 and SW5 by switch control signal CTL3, second line memory 42 is accessed. The sampled data at the location of ".beta." is stored in second line memory 42 through switch SW4, and then is supplied to intermediate gradation circuit 19 and TpH 20 through switch SW5, so that the color signal data of a second line is printed on the paper.
As described above, the sampled data at a plurality of location is sequentially printed. Thus, in effect, an entire picture defined only by color signal B is printed, line by line, on the print paper by this process. Generally, 500 to 600 lines of data are printed in this manner in a single color.
After printing with respect to the B signal is accomplished, switch SW3 receives the G signal through input f by switch control signal CTL2. Then, the G signal supplied to A/D converter 18 through switch SW3 is printed through the same procedure as in the case of the B signal. After printing with respect to the G signal is accomplished, switch SW3 receives the R signal through input g by switch control signal CTL2. Then, the R signal is printed through the same procedure as in the case of the B and G signals. In this manner, the respective color signals are printed in turn, so that all color signal data in one picture screen is printed. Here, colors of Yellow, Magenta and Cyan are printed by the B, G and R signals, respectively.
However, in such a conventional color video printer, when the images which are photographed toward the sun are produced and printed, the whole printed images have the tendency of growing dark.